1. Field of the Invention
This invention relates to apparatus for on-line monitoring of three-phase ac motors for stator electrical faults, and in particular, to such apparatus which uses a single channel to monitor all three phases, yet detects all series and parallel electrical faults related to the stator.
2. Background Information
Induction motor faults can be conveniently divided into stator, rotor, bearing, and other failures. A fault may progress from one type to another, for instance, rotor to bearing. A common physical process is stator insulation breakdown, which greatly contributes to various stator winding failures: winding/winding, phase/phase, phase/floating neutral, and phase/ground. In addition, series faults can result from a variety of causes: loose connections, breaks in fatigued wires, and the like. Vibration and contamination, often present in industrial motor environments, contribute to and exacerbate the effects of arcing, either in series or parallel. High temperatures from overloads or the environment accelerate insulation wear and breakdown.
Common solutions to motor diagnostics involve off-line measurement using testers, or on-line techniques using high resolution Fourier analysis at low frequencies of phenomena other than arcing. Moreover, faults which develop on continuously running motors which cannot conveniently be taken out of service might not be detected before a catastrophic failure. Another problem is the percentage of motors, albeit small, in which the windings are actually stressed during the testing.
A proposal has been made for on-line monitoring of ac motors for discharge activity. "On-Line Discharge Measurements on Motors", K. G. Barnley, The 6th BEAMA International Electrical Insulation Conference, May 1990. Sensed currents generated by current transducers placed around each of the supply cables are analyzed in separate signal processing channels. A digital readout of the magnitude and repetition rate of pulses generated by discharge activity is presented. While it is theorized that similar measurements could be made on motors with a single three-phase supply cable, no suggestion is provided as to how this could be accomplished. A single transducer would only be able to detect ground faults.
It is known to monitor the current in the neutral conductor of a wye connected three-phase motor, as imbalances in the phase currents are reflected in the neutral. However, most wye connected motors are not used in supply systems with a neutral conductor so that the neutral connection floats.
There is a need therefore for improved apparatus for on-line monitoring of stator electrical faults in three-phase ac motors.
In particular there is a need for such improved apparatus which is simple and moderate in cost.
More specifically, there is a need for such improved apparatus which can detect all types of stator electrical faults with single channel processing, without, for example, resorting to the more complex strategy of multiplexing.
There is an additional need for such apparatus which can be used with delta connected motors and wye connected motors with a floating neutral.